The invention relates in general to a polymeric orthopaedic bearing material for use as an implant or an implant part, and in particular to an orthopaedic bearing material made of ultrahigh molecular weight polyethylene (UHMWPE). The bearing material has an advantageous combination of wear or abrasion resistance and toughness.
Implants have been used to replace parts of the human body, e.g., the hip, the knee, and the extremity joints. In an implant, the bearing material having a bearing surface is paired with an opposing metal or ceramic component. Polymeric materials, for example, UHMWPE, have been used for producing bearing materials. The high molecular weight of the polymer imparts certain desirable characteristics such as high impact strength and abrasion resistance. However, the bearing material, even when made of UHMWPE, wears due to use over time and, as a result, introduces debris, e.g., microscopic wear particles, into the surrounding tissues. The body's reactions to the debris can involve inflammation and deterioration of the affected tissues. In particular, the bone to which the prosthesis or implant is anchored can deteriorate and become inflamed. Eventually, the prosthesis may become painfully loose in which case it must be replaced. It is generally accepted by orthopaedic surgeons and biomaterials scientists that the reaction of the affected tissue to wear debris is one of the major causes of long-term failure of such prostheses.
Crosslinking of the polymer, e.g., by the use of ionizing radiation such as gamma radiation or e-beam or of plasma gases, has been proposed for improving the wear resistance of implants. While crosslinking improves the wear resistance, it also tends to lower one or more of the desirable mechanical properties. In particular, crosslinking tends to lower the toughness of the bearing material resulting in a greater susceptibility to mechanical failure of the implant. Crosslinking, for example, by irradiation, can also change the color of the bearing material. It is believed that the change in color and/or mechanical properties may be due to oxidation of the polymer chains leading to a decrease in polymer molecular weight. The ionizing radiation breaks molecular bonds, resulting in polymer chain scission, and creates free radicals that are highly reactive species. With the passage of time, the severed chains can recombine, crosslink with adjacent chains, or combine with other species such as oxygen. In the presence of oxygen, the severed chain is also likely to form an oxygenated species which is then not able to form crosslinks or recombine, resulting in a reduction of molecular weight. It is believed that the reduction in molecular weight causes a reduction in one or more mechanical properties, e.g., toughness, impact strength, or tensile strength, and can in severe cases cause embrittlement.
Further, some of the free radicals formed during irradiation are not capable of reacting in a short time frame due to their remote location within the polymer structure and thus can exist as isolated free radicals for long periods of time. These isolated free radicals are therefore difficult to neutralize during the manufacture of the bearing material. However, even these free radicals eventually react as a result of migration of oxygen or other reactive species to such remote locations over time, which can also lead to time-dependent degradation of the properties.
Approaches have been pursued to stabilize or neutralize the free radicals. For example, in one approach, the use of vitamin E has been proposed to stabilize the free radicals. Thus, a vitamin E-treated UHMWPE bearing material is irradiated to obtain a crosslinked bearing material. A disadvantage of this approach is that the entire bearing material, i.e., the surface and the body, is irradiated. As a result, some of the mechanical properties of the bearing material are compromised. In addition, the presence of vitamin E in an irradiated polymer can impart a strong yellow-brown color to the material which is aesthetically undesirable.
In another approach, it has been proposed to crosslink the entire articular surface of the bearing material. Since the crosslinked surface layer has reduced impact strength, a disadvantage of this approach is that the crosslinked surface layer could be damaged or worn through relatively easily, thereby exposing the underlying polymer which has a poor wear resistance. In addition, the surface layer can act as an initiation point for other mechanical failures.
In yet another approach, UHMWPE powder is irradiated to obtain a crosslinked powder, which is then blended with non-crosslinked UHMWPE powder, and the resulting blend is molded into a bearing material. This approach produces a bearing material, which contains throughout its thickness, discrete crosslinked polymer domains or particles surrounded by a non-crosslinked matrix. A disadvantage of this approach is that it can be difficult to obtain good adhesion between the crosslinked particles or domains and the non-crosslinked particles or matrix. If the proportion of the crosslinked powder is increased to obtain improvement in wear resistance, some of the mechanical properties of the resulting bearing material could become poor.
The foregoing shows that there exists a need for bearing materials having an advantageous combination of at least one wear property and at least one mechanical property. The invention provides such a bearing material.